Pixelitor is a Java/cross-platform image editor. It provides image filters, drawing tools, layered editing, bleding modes, multiple undo steps, etc. Its main attraction are the more than 80 unique image filters and color adjustment tools. Besides supporting standard graphic files, it defaults to saving in its custom PXC format (a gzipped Java object serialization for retaining layer data).
Oneshot is a very simple picture editor. It provides few features, but line drawing, adding arrows or circles, boxes and callouts to photograps. It's intended for unversed users; supports copying+pasting, or clipboard exchange however.